B001 - Atmega16 - Assembly - Address space allocation

Address space planning

Open m16def.inc and you can see the following definition, which corresponds one-to-one with the address space of FlashROM, SRAM, EEPROM, etc. described in the manual.

; ***** DATA MEMORY DECLARATIONS *****************************************

.equ FLASHEND = 0x1fff ; Note: Word address, here is a total of 8K word flash ROM

.equ IOEND     = 0x003f ; The address space of IO registers is 0x0000-0x003F, but they are mapped to 0x0020-0x005F

.equ SRAM_START = 0x0060; 0x0060-0x0045F This 1KB address space is the address space used by the internal RAM

.equ SRAM_SIZE = 1024;

.equ RAMEND = 0x045f;

.equ XRAMEND = 0x0000 ; This may be the address space reserved for external RAM - question-001

.equ E2END    = 0x01ff; EEPROM has a total of 512B, occupying the address space of 0x0000-0x01FF

.equ EEPROMEND = 0x01ff; This address is sent to the EEPROM control register for use

.equ EEADRBITS = 9 ; 512 cells require 9 address lines to access

#pragma AVRPART MEMORY PROG_FLASH 16384 ; 8K word = 16K byte

#pragma AVRPART MEMORY EEPROM 512

#pragma AVRPART MEMORY INT_SRAM SIZE 1024

#pragma AVRPART MEMORY INT_SRAM START_ADDR 0x60

After the compiler is finished compiling, the usage of FlashROM (.cseg), SRAM (.dseg), and EEPROM (.eseg) will be given:

illustrate:

1. When using the in and out instructions to access IO registers, these two instructions use the address space of 0x0000-0x003F to access IO registers.

2. When using ld and st instructions to access IO registers, you need to use the mapped address space 0x0020-0x005F ([0x0000, 0x003F]+0x0020) to access IO registers.

       IO registers such as PORTB and SREG are defined in the m16def.inc file according to the address space of 0x0000-0x003F.

       Therefore, when using the ld and st instructions, the register address needs to be added with 0x0020: PORTB + 0x20, SREG + 0x20.

Stack space in RAM

0. The stack (actually refers to the stack) is also part of the RAM, but it can be accessed in FILO form using special instructions push/pop and the stack pointer SP (the address automatically increases or decreases in SP).

      In DEBUG mode, we can see that the SP pointer is placed at the end of the RAM, growing downwards from the back.

1. In the reset interrupt service routine, the stack pointer SP is set to 0x045F, which is the last unit of the RAM address space:

RAMEND is defined in m16def.inc as follows (part of the above code):

.equ RAMEND = 0x045f

2. The push instruction saves the data in R20 to the RAM unit pointed to by the SP pointer, that is, pushes it onto the stack:

3. Pop instruction, take out the contents of the RAM unit pointed to by SP pointer and put it into R20, that is, pop the stack: